Metal-coated steel strip

ABSTRACT

An Al—Zn—Si—Mg alloy coated strip that has Mg2Si particles in the coating microstructure is disclosed. The distribution of Mg2Si particles is such that a surface region of the coating has only a small proportion of Mg2Si particles or is at least substantially free of any Mg2Si particles.

The present invention relates to strip, typically steel strip, which has a corrosion-resistant metal alloy coating.

The present invention relates particularly to a corrosion-resistant metal alloy coating that contains aluminium-zinc-silicon-magnesium as the main elements in the alloy, and is hereinafter referred to as an “Al—Zn—Si—Mg alloy” on this basis. The alloy coating may contain other elements that are present as deliberate alloying additions or as unavoidable impurities. Hence, the phrase “Al—Zn—Si—Mg alloy” is understood to cover alloys that contain such other elements and the other elements may be deliberate alloying additions or unavoidable impurities.

The present invention relates particularly but not exclusively to steel strip that is coated with the above-described Al—Zn—Si—Mg alloy and can be cold formed (e.g. by roll forming) into an end-use product, such as roofing products.

Typically, the Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium:

-   -   Aluminium: 40 to 60%     -   Zinc: 40 to 60%     -   Silicon: 0.3 to 3%     -   Magnesium 0.3 to 10%

Typically, the corrosion-resistant metal alloy coating is formed on steel strip by a hot dip coating method.

In the conventional hot-dip metal coating method, steel strip generally passes through one or more heat treatment furnaces and thereafter into and through a bath of molten metal alloy held in a coating pot. The heat treatment furnace that is adjacent a coating pot has an outlet snout that extends downwardly to a location below the upper surface of the bath.

The metal alloy is usually maintained molten in the coating pot by the use of heating inductors. The strip usually exits the heat treatment furnaces via an outlet end section in the form of an elongated furnace exit chute or snout that dips into the bath. Within the bath the strip passes around one or more sink rolls and is taken upwardly out of the bath and is coated with the metal alloy as it passes through the bath.

After leaving the coating bath the metal alloy coated strip passes through a coating thickness control station, such as a gas knife or gas wiping station, at which its coated surfaces are subjected to jets of wiping, gas to control the thickness of the coating.

The metal alloy coated strip then passes through a cooling section and is subjected to forced cooling.

The cooled metal alloy coated strip may thereafter be optionally conditioned by passing the coated strip successively through a skin pass rolling section (also known as a temper rolling section) and a tension levelling section. The conditioned strip is coiled at a coiling station.

A 55% Al—Zn alloy coating is a well known metal alloy coating for steel strip. After solidification, a 55% Al—Zn alloy coating normally consists of α-Al dendrites and a β-Zn phase in the inter-dendritic regions of the coating.

It is known to add silicon to the coating alloy composition to prevent excessive alloying between the steel substrate and the molten coating in the hot-dip coating method. A portion of the silicon takes part in a quaternary alloy layer formation but the majority of the silicon precipitates as needle-like, pure silicon particles during solidification. These needle-like silicon particles are also present in the inter-dendritic regions of the coating.

It has been found by the applicant that when Mg is included in a 55% Al—Zn—Si alloy coating composition, Mg brings about certain beneficial effects on product performance, such as improved cut-edge protection, by changing the nature of corrosion products formed.

However, it has also been found by the applicant that Mg reacts with Si to form a Mg₂Si phase and that the formation of the Mg₂Si phase compromises the above-mentioned beneficial effects of Mg in a number of ways.

By way of example, the Mg₂Si phase forms as large particles in relation to typical coating thicknesses and can provide a path for rapid corrosion where particles extend from a coating surface to an alloy layer adjacent the steel strip.

By way of further example, the Mg₂Si particles tend to be brittle and sharp particles and provide both an initiation and propagation path for cracks that form on bending of coated products formed from coated strip. Increased cracking compared to Mg-free coatings can result in more rapid corrosion of the coatings.

The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

The present invention is an Al—Zn—Si—Mg alloy coated strip that has Mg₂Si particles in the coating microstructure with the distribution of Mg₂Si particles being such that a surface region of the coating has only a small proportion of Mg₂Si particles or is at least substantially free of any Mg₂Si particles.

The term “surface region” is understood herein to mean a region that extends inwardly from the exposed surface of a coating.

The applicant has found that the above-described distribution of Mg₂Si particles in the coating microstructure provides significant advantages and can be achieved by any one or more of:

-   -   (a) strontium additions in the coating alloy;     -   (b) selection of the cooling rate during solidification of         coated strip for a given coating mass (i.e. coating thickness)         exiting a coating bath; and     -   (c) minimising variations in coating thickness.

According to the present invention there is provided an Al—Zn—Si—Mg alloy coated steel strip that comprises a coating of an Al—Zn—Si—Mg alloy on a steel strip, with the microstructure of the coating comprising Mg₂Si particles, and with the distribution of the Mg₂Si particles being such that there is only a small proportion of Mg₂Si particles or at least substantially no Mg₂Si particles in a surface region of the coating.

The small proportion of Mg₂Si particles in the surface region of the coating may be no more than 10 wt. % of the Mg₂Si particles.

Typically, the Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium:

-   -   Aluminium: 40 to 60%     -   Zinc: 40 to 60%     -   Silicon: 0.3 to 3%     -   Magnesium 0.3 to 10%

The Al—Zn—Si—Mg alloy may also contain other elements, such as, by way of example any one or more of iron, vanadium, chromium, and strontium.

Preferably the surface region has a thickness that is at least 5% of the total thickness of the coating.

Preferably the surface region has a thickness that is less than 30% of the total thickness of the coating.

More preferably the surface region has a thickness that is less than 20% of the total thickness of the coating.

More preferably the surface region has a thickness that is 5-30% of the total thickness of the coating.

Preferably at least a substantial proportion of the Mg₂Si particles are in a central region of the coating.

The substantial proportion of the Mg₂Si particles in the central region of the coating may be at least 80 wt. % of the Mg₂Si particles.

Typically, the coating thickness is less than 30 μm.

Preferably the coating thickness is greater than 7 μm.

The coating microstructure may also include a region that is adjacent the steel strip that has only a small proportion of Mg₂Si particles or is at least substantially free of any Mg₂Si particles, whereby the Mg₂Si particles in the coating microstructure are at least substantially confined to a central or core region of the coating.

Preferably the coating contains more than 250 ppm Sr, with the Sr addition promoting the formation of the above distribution of Mg₂Si particles in the coating.

Preferably the coating contains more than 500 ppm Sr.

Preferably the coating contains more than 1000 ppm Sr.

Preferably the coating contains less than 3000 ppm Sr.

The Al—Zn—Si—Mg—Sr alloy coating may contain other elements as deliberate additions or as unavoidable impurities.

Preferably there are minimal coating thickness variations.

According to the present invention there is also provided a hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, Mg, and more than 250 ppm Sr and optionally other elements and forming an alloy coating on the strip that has Mg₂Si particles in the coating microstructure with the distribution of the Mg₂Si particles being such that there is only a small proportion of Mg₂Si particles or substantially no Mg₂Si particles in a surface region of the coating.

Preferably the coating contains more than 500 ppm Sr.

Preferably the coating contains more than 1000 ppm Sr.

Preferably the molten bath contains less than 3000 ppm Sr.

The Al—Zn—Si—Mg—Sr alloy coating may contain other elements as deliberate additions or as unavoidable impurities.

According to the present invention there is also provided a hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip, and cooling coated strip exiting the coating bath during solidification of the coating at a rate that is controlled so that the distribution of Mg₂Si particles in the coating microstructure is such that there is only a small proportion of Mg₂Si particles or substantially no Mg₂Si particles in a surface region of the coating.

The small proportion of Mg₂Si particles in the surface region of the coating may be no more than 10 wt. % of the Mg₂Si particles.

Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to be at less than a threshold cooling rate.

In any given situation, the selection of the required cooling rate is related to the coating thickness (or coating mass).

Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 80° C./sec for coating masses up to 75 grams per square metre of strip surface per side.

Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 50° C./sec for coating masses 75-100 grams per square metre of strip surface per side.

Typically, the method comprises selecting the cooling rate for coated strip exiting the coating bath to at least 11° C./sec.

The coating bath and the coating on steel strip coated in the bath may contain Sr.

According to the present invention there is also provided a hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip with minimal variation in the thickness of the coating so that the distribution of Mg₂Si particles in the coating microstructure is such that there is only a small proportion of Mg₂Si particles or substantially no Mg₂Si particles in a surface region of the coating.

Preferably the coating thickness variation should be no more than 40% in any given 5 mm diameter section of the coating.

More preferably the coating thickness variation should be no more than 30% in any given 5 mm diameter section of the coating.

In any given situation, the selection of an appropriate thickness variation is related to the coating thickness (or coating mass).

By way of example, for a coating thickness of 22 μm, preferably the maximum thickness in any given 5 mm diameter section of the coating should be 27 μm.

Preferably the method comprises selecting the cooling rate during solidification of coated strip exiting the coating bath to be less than a threshold cooling rate.

The coating bath and the coating on steel strip coated in the bath may contain Sr.

The hot-dip coating method may be the conventional method described above or any other suitable method.

The advantages of the invention include the following advantages.

-   -   Enhanced corrosion resistance. The Mg₂Si distribution of the         present invention eliminates direct corrosion channels from the         coating surface to steel strip that occurs with a conventional         Mg₂Si distribution. As a result, the corrosion resistance of the         coating is markedly enhanced.     -   Improved coating ductility. Mg₂Si particles at the coating         surface and adjacent to the steel strip are effective crack         initiation sites when the coating undergoes a high strain         fabrication. The Mg₂Si distribution of the present invention         eliminates such crack initiation sites altogether or         substantially reduces the total number of crack initiation         sites, resulting in a significantly improved coating ductility.     -   The addition of Sr allows the use of higher cooling rates,         reducing the length of cooling equipment required after the pot.

EXAMPLE

The applicant has carried out laboratory experiments on a series of 55% Al—Zn-1.5% Si-2.0% Mg alloy compositions having up to 3000 ppm Sr coated on steel substrates.

The purpose of these experiments was to investigate the impact of Sr on the distribution of Mg₂Si particles in the coatings.

FIG. 1 summarises the results of one set of experiments carried out by the applicant that illustrate the present invention.

The left hand side of the FIGURE comprises a top plan view of a coated steel substrate and a cross-section through the coating with the coating comprising a 55% Al—Zn-1.5% Si-2.0% Mg alloy with no Sr. The coating was not formed having regard to the selection of cooling rate during solidification discussed above.

It is evident from the cross-section that Mg₂Si particles are distributed throughout the coating thickness. This is a problem for the reasons stated above.

The right hand side of the FIGURE comprises a top plan view of a coated steel substrate and a cross-section through the coating, with the coating comprising a 55% Al—Zn-1.5% Si-2.0% Mg alloy and 500 ppm Sr. The cross-section illustrates upper and lower regions at the coating surface and at the interface with the steel substrate that are completely free of Mg₂Si particles, with the Mg₂Si particles being confined to a central band of the coating. This is advantageous for the reasons stated above.

The photomicrographs of the FIGURE illustrate clearly the benefits of the addition of Sr to an Al—Zn—Si—Mg coating alloy.

The laboratory experiments found that the microstructure shown in the right hand side of the FIGURE were formed with Sr additions in the range of 250-3000 ppm.

The applicant has also carried out line trials on 55% Al—Zn-1.5% Si-2.0% Mg alloy composition (not containing Sr) coated on steel strip.

The purpose of these trials was to investigate the impact of cooling rates and coating masses on the distribution of Mg₂Si particles in the coatings.

The experiments covered a range of coating masses from 60 to 100 grams per square metre surface per side of strip, with cooling rates up to 90° C./sec.

The applicant found two factors that affected the coating microstructure, particularly the distribution of Mg₂Si particles in the coatings.

The first factor is the effect of the cooling rate of the strip exiting the coating bath before completing the coating solidification. The applicant found that controlling the cooling rate is important.

By way of example, the applicant found that for a AZ150 class coating (or 75 grams of coating per square metre surface per side of strip—refer to Australia Standard AS1397-2001), if the cooling rate is greater than 80° C./sec, Mg₂Si particles formed in the surface region of the coating.

The applicant also found that for the same coating it is not desirable that the cooling rate be too low, particularly below 11° C./sec, as in this case the coating develops a defective “bamboo” structure, whereby the zinc-rich phases forms a vertically straight corrosion path from the coating surface to the steel interface, which compromises the corrosion performance of the coating.

Therefore, for a AZ150 class coating, under the experimental conditions tested, the cooling rate should be controlled to be less than 80° C./sec and typically in a range of 11-80° C./sec.

On the other hand, the applicant also found that for a AZ200 class coating, if the cooling rate was greater than 50° C./sec, Mg₂Si particles formed on the surface of the coating.

Therefore, for a AZ200 class coating, under the experimental conditions tested, a cooling rate of less than 50° C./sec and typically in a range of 11-50° C./sec is desirable.

The research work carried out by the applicant on the solidification of Al—Zn—Si—Mg coatings, which is extensive and is described in part above, has helped the applicant to develop an understanding of the formation of the Mg₂Si phase in a coating and the factors affecting its distribution in the coating. Whilst the applicant does not wish to be bound by the following discussion, this understanding is as set out below.

When an Al—Zn—Si—Mg alloy coating is cooled to a temperature in the vicinity of 560° C., the α-Al phase is the first phase to nucleate. The α-Al phase then grows into a dendritic form. As the α-Al phase grows, Mg and Si, along with other solute elements, are rejected into the molten liquid phase and thus the remaining molten liquid in the interdendritic regions is enriched in Mg and Si.

When the enrichment of Mg and Si in the interdendritic regions reaches a certain level, the Mg₂Si phase starts to form, which also corresponds to a temperature around 465° C. For simplification, it will be assumed that an interdendritic region near the outer surface of the coating is region A and another interdendritic region near the quaternary intermetallic alloy layer at the steel strip surface is region B. It will also be assumed that the level of enrichment in Mg and Si is the same in region A as in region B.

At or below 465° C., the Mg₂Si phase has the same tendency to nucleate in region A as in region B. However, the principles of physical metallurgy teach us that a new phase will preferably nucleate at a site whereupon the resultant system free energy is the minimum. The Mg₂Si phase would normally nucleate preferably on the quaternary intermetallic alloy layer in region B provided the coating bath does not contain Sr (the role of Sr with Sr-containing coatings is discussed below). The applicant believes that this is in accordance with the principles stated above, in that there is a certain similarity in crystal lattice structure between the quaternary intermetallic alloy phase and the Mg₂Si phase, which favours the nucleation of Mg₂Si phase by minimizing any increase in system free energy. In comparison, for the Mg₂Si phase to nucleate on the surface oxide of the coating in region A, the increase in system free energy would have been greater.

Upon nucleation in region B, the Mg₂Si phase grows upwardly, along the molten liquid channels in the interdendritic regions, towards region A. At the growth front of the Mg₂Si phase (region C), the molten liquid phase becomes depleted in Mg and Si (depending on the partition coefficients of Mg and Si between the liquid phase and the Mg₂Si phase), compared with that in region A. Thus a diffusion couple forms between region A and region C. In other words, Mg and Si in the molten liquid phase will diffuse from region A to region C. Note that the growth of the α-Al phase in region A means that region A is always enriched in Mg and Si and the tendency for the Mg₂Si phase to nucleate in region A always exists because the liquid phase is “undercooled” with regard to the Mg₂Si phase.

Whether the Mg₂Si phase is to nucleate in region A, or Mg and Si are to keep diffusing from region A to region C, will depend on the level of Mg and Si enrichment in region A, relevant to the local temperature, which in turn depends on the balance between the amount of Mg and Si being rejected into that region by the α-Al growth and the amount of Mg and Si being moved away from that region by the diffusion. The time available for the diffusion is also limited, as the Mg₂Si nucleation/growth process has to be completed at a temperature around 380° C., before the L→Al—Zn eutectic reaction takes place, wherein L depicts the molten liquid phase.

The applicant has found that controlling this balance can control the subsequent nucleation or growth of the Mg₂Si phase or the final distribution of the Mg₂Si phase in the coating thickness direction.

In particular, the applicant has found that for a set coating thickness, the cooling rate should be regulated to a particular range, and more particularly not to exceed a threshold temperature, to avoid the risk for the Mg₂Si phase to nucleate in region A. This is because for a set coating thickness (or a relatively constant diffusion distance between regions A and C), a higher cooling rate will drive the α-Al phase to grow faster, resulting in more Mg and Si being rejected into the liquid phase in region A and a greater enrichment of Mg and Si, or a higher risk for the Mg₂Si phase to nucleate, in region A (which is undesirable).

On the other hand, for a set cooling rate, a thicker coating (or a thicker local coating region) will increase the diffusion distance between region A and region C, resulting in a smaller amount of Mg and Si being able to move from region A to region C by the diffusion within a set time and in turn a greater enrichment of Mg and Si, or a higher risk for the Mg₂Si phase to nucleate, in region A (which is undesirable).

Practically, the applicant has found that, to achieve the distribution of Mg₂Si particles of the present invention, i.e. to avoid nucleation of the Mg₂Si phase in region A, the cooling rate for coated strip exiting the coating bath has to be in a range of 11-80° C./sec for coating masses up to 75 grams per square metre of strip surface per side and in a range 11-50° C./sec for coating masses of 75-100 grams per square metre of strip surface per side. The short range coating thickness variation also has to be controlled to be no greater than 40% above the nominal coating thickness within a distance of 5 mm across the strip surface to achieve the distribution of Mg₂Si particles of the present invention.

The applicant has also found that, when Sr is present in a coating bath, the above described kinetics of Mg₂Si nucleation can be significantly influenced. At certain Sr concentration levels, Sr strongly segregates into the quaternary alloy layer (i.e. changes the chemistry of the quaternary alloy phase). Sr also changes the characteristics of surface oxidation of the molten coating, resulting in a thinner surface oxide on the coating surface. Such changes alter significantly the preferential nucleation sites for the Mg₂Si phase and, as a result, the distribution pattern of the Mg₂Si phase in the coating thickness direction. In particular, the applicant has found that, Sr at concentrations 250-3000 ppm in the coating bath makes it virtually impossible for the Mg₂Si phase to nucleate on the quaternary alloy layer or on the surface oxide, presumably due to the very high level of increase in system free energy would otherwise be generated. Instead, the Mg₂Si phase can only nucleate at the central region of the coating in the thickness direction, resulting in a coating structure that is substantially free of Mg₂Si at both the coating outer surface region and the region near the steel surface. Therefore, Sr additions in the range 250-3000 ppm are proposed as one of the effective means to achieve a desired distribution of Mg₂Si particles in a coating.

Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.

In this context, whilst the above description of the present invention focuses on (a) the addition of Sr to Al—Zn—Si—Mg coating alloys, (b) regulating cooling rates (for a given coating mass) and (c) minimising variations in coating thickness as means for achieving a desired distribution of Mg₂Si particles in coatings, i.e. at least substantially no Mg₂Si particles in the surface of a coating, the present invention is not so limited and extends to the use of any suitable means to achieve the desired distribution of Mg₂Si particles in the coating. 

1.-28. (canceled)
 29. A hot-dip coating method for forming a corrosion-resistant and cracking-resistant Al—Zn—Si—Mg alloy coated steel strip, the method comprising: passing a steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg to form an alloy coating on the steel strip, the alloy coating comprising, in weight %, 40 to 60% aluminum, 40 to 60% zinc, 0.3 to 3% silicon, 0.3 to 10% magnesium; passing the steel strip with alloy coating through a coating thickness control station and controlling a short range coating thickness variation of the alloy coating on the steel strip to be no greater than 40% in a 5 mm diameter section of the coating; and cooling the alloy coating to form the Al—Zn—Si—Mg alloy coated steel strip; wherein the alloy coating comprises Mg₂Si particles, and wherein controlling the short range coating thickness variation controls a microstructure distribution of the Mg₂Si particles in the alloy coating such that there is no more than 10 wt. % of the Mg₂Si particles in a surface region of the coating to form a corrosion-resistant and cracking-resistant Al—Zn—Si—Mg alloy coated steel strip.
 30. The method of claim 29, wherein the coating thickness variation is no more than 30% in a 5 mm diameter section of the coating.
 31. The method of claim 29, wherein passing the steel strip with alloy coating through the coating thickness control station further controls a coating thickness of the alloy coating to be greater than 7 μm and less than 30 μm.
 32. The method of claim 29, wherein the hot dip coating bath further comprises one or more of strontium, iron, vanadium, and chromium, and the alloy coating comprises one or more of strontium, iron, vanadium, and chromium.
 33. The method of claim 29, wherein the surface region of the coating comprises substantially no Mg₂Si particles.
 34. The method of claim 29, wherein the surface region of the coating comprises no Mg₂Si particles.
 35. The method of claim 29, wherein the coating thickness control station is a gas knife or gas wiping station.
 36. The method of claim 29, wherein cooling the alloy coating occurs at a cooling rate of less than 80° C./sec and greater than 11° C./sec for coating masses up to 75 grams per square meter of strip surface per side, or at a cooling rate of less than 50° C./sec and greater than 11° C./sec for coating masses 75 to 100 grams per square meter of strip surface per side. 